This invention relates to interleafed fiber resin matrix materials. More particularly, the invention relates to improved interleafed fiber resin matrix fabric sheeting having high impact strength.
High strength to weight materials constructed from fiber resin matrix composites have become widely used in many industries where strength, corrosion resistance and light weight are desirable. For example, resin matrix materials are particularly well suited for use in aircraft structural members. Sporting equipment, such as tennis rackets and golf clubs, has also been successfully constructed from such materials.
Improved composites having high impact strength and resistance to impact damage are described in commonly assigned, copending U.S. application Ser. No. 572,263, filed Jan. 19, 1984, which is hereby incorporated by reference. Bi-layer composites are described therein which comprise (A) a fiber resin matrix layer of high-strength reinforcing filaments coated with a thermosetting epoxy resin and (B) a discrete interleaf layer of modified thermosetting epoxy resin, containing a rubbery, extensible polymer material. The composites are commonly formed into prepreg tapes, wherein the filaments are arranged in a flat, unidirectional array, and the tapes used to build, or "lay-up", a structural laminate, where the interleaf layers provide special advantages in terms of shear and impact strength.
When the reinforcing filaments are in the configuration of a fabric, however, great difficulty has been encountered in maintaining a continuous interleaf layer. When the bi-layer prepreg fabrics are laminated and cured under pressure, the thin interleaf resin layer is broken up, largely eliminating any improvements in performance. This is at least partially due to the construction of reinforcing fabrics: Fiber bundles, or "tows", of about 1.000 to 12,000 individual fibers are interwoven to form a high-strength fabric sheet, which is in turn coated and impregnated with thermosetting resin. On a microscopic level, the surface of such a fabric is very uneven and a thin interleaf layer, when pressed between fabric sheets and cured, is easily disrupted.
Attempting to preserve the interleaf layer by changing the curing process or radically altering the interleaf layer are imperfect solutions: Changes in the curing conditions or chemistry increases the cost, time and expertise required and often result in inferior composites; applying a thicker layer of interleaf resin or reinforcing it also adds cost but also, and much more importantly, abrogates the high strength-to-weight ratio that gives fiber resin matrix materials their advantage over metals.
SUMMARY OF THE INVENTION
It has now been discovered that incorporating a thin, non-woven or woven fibrous mat or carrier into the elastomeric interleaf resin layer sufficiently supports the interleaf during cure to provide a fiber resin matrix prepreg fabric having significantly increased impact resistance, without significantly adding to the gross weight of the final composite. The non-woven or woven fibrous mat or carrier comprising, e.g., lightweight glass, polyester, graphite, carbon, boron, polyaramide, cellulosic, etc. fibers, does not reinforce the interleaf or add strength to the ultimate composite, rather it supports the interleaf resin during cure, providing a final interleaf layer that is continuous and integral. The preserved continuous interleaf layer in turn provides finished composites having greater impact strength and resistance to delamination after impact damage.
Accordingly, it is an object of the present invention to provide an interleafed fiber resin matrix prepreg fabric exhibiting improved impact resistance, or "toughness", over known prior art resin matrix materials.
It is a further object of the present invention to provide a cured interleafed fabric resin matrix composite having continuous, integral interleaf resin layers.
It is a further object of the present invention to provide a method for producing interleafed fiber resin matrix prepreg fabric having a discrete, integral resin interleaf layer and exhibiting improved impact strength after cure.
These and other objects are accomplished herein by a method for producing an interleafed prepreg fabric comprising
(1) incorporating a lightweight fibrous mat or carrier of supporting filaments into an interleaf resin composition comprising (a) a first thermosetting epoxy resin composition containing (b) about 8% to 70% by weight of a rubbery vinyl addition polymer to provide a supported interleaf resin layer, and
(2) introducing said supported interleaf resin layer to at least one surface of a fiber resin matrix comprising (a) a fabric of high-strength reinforcing filaments and (b) a second thermosetting epoxy resin composition.
Also contemplated are prepreg fabrics comprising
(A) a fiber resin matrix layer comprising (i) interwoven high-strength reinforcing filaments and (ii) a thermosetting epoxy resin composition coating said interwoven reinforcing filaments, and
(B) a discrete interleaf resin layer comprising (i) a lightweight fibrous mat or carrier of supporting filaments and (ii) a modified thermosetting epoxy resin composition containing from about 8% to 70% by weight of a rubbery vinyl addition polymer.